This invention relates to a system for correcting shading or non-uniformity which occurs in the output of an array of photosensitive elements used in picture image apparatus such as electric printers, facsimile or optical character recognition (OCR) systems.
The phenomenon called shading occurs in the output of photosensitive elements. In accordance with this phenomenon the photosensitive array produces an output which varies as the position of the impinging light along the surface of the photoarray. FIG. 1 shows the characteristics of a linear sensor array for a given image tone. As shown in FIG. 1, the intensity of output signal will be higher at the center of the sensor array than at its edges. This produces undesirable results in the reproduced printed copy. That is, the original is inaccurately read and the printed copy has intensity graduations resulting in inaccurate reproduction of the original. In some cases, the output will vary more randomly due to the inherent nonuniform characteristics or sensitivity among the various array elements. This is a further factor causing an undesirable output.
There are three recognized causes of shading or non-uniformities of sensor output: array elements having non-uniform sensitivity; non-uniformity of certain light sources (e.g., fluorescent lights); and inherent characteristics of lens systems to vary light intensity along surface of array. In particular, in constructing a sensor array it is virtually impossible to produce a multitude of sensor elements each having identical output characteristics; as a result, the output of each element will be different for a given image tone and intensity of impinging light. The resulting output has a random intensity depending upon the characteristic of each adjacent element even though each sensor may receive light from the same image tone. Without some correction, the resulting printed copy will have graduations in intensity and will be commercially undesirable.
The second cause of non-uniformity are certain light sources. For example, fluorescent light sources produce an output intensity which is constant at its center portion but is substantially reduced at its edges. While attempts have been made to use a very long fluorescent source whereby only the centermost region supplies the desired light, this has necessarily resulted in a very large and impractical reproduction facsimile system. Some light sources, moreover, have characteristics which change due to sputtering of the filament caused by aging. As a result, the output of the source will vary nonuniformily along its length to a greater degree, producing even more severe degradation in reproduced printed copy.
Finally, in some reproduction system, a lens system is used to focus the light image on the array. Such systems, however, compounded the non-uniformity problem discussed above due to the inherent characteristics of lenses. That is, a lens has the characteristic of transmitting light of varying intensity along its surface (See, e.g., Optics by Hiroshi Kubota, Iwanami Shoten Publishing Co., Toyoko, September 1975).
One prior art method provides some correction for the characteristics shown in FIG. 1 by utilizing a correcting board positioned between the document and the lens system. The board is opaque and has a particular shape to gradually prevent passage of light from the centermost portion of the document to the detector. Consequently, the more intense light at the center will be reduced to substantially equal the light intensity at the edges of the detector. In attempting to achieve uniformity of light with a correcting board, it must be specially designed and oriented differently for each case since the characteristics of each light source is different. Practically, however, the board design has never achieved complete uniformity of output intensity for a given tone image and has required the costly and time consuming process, during manufacturing of adjusting and orienting each board.
Another prior art method utilizes a lamp shading plate or cover which corrects for the non-uniformity of the lamp (see, Japanese Utility Model Publication No. 53-15211, Apr. 21, 1978). This method, however, does not correct for non-uniformity of the photoelements or aging of the lamp.
A further prior art method employs an opaque slotted lamp cover plate (see, e.g., Japanese Utility Model Publication No. 52-16121, Apr. 12, 1977). The width of the slots are varied to correct for the non-uniformity light intensity of the lamp. this method does not correct for non-uniformity of the photoelements or aging of the lamp.
A still further prior art method is disclosed in Japanese Utility Model Publication No. 28-12984, Dec. 28, 1953. This method utilizes two spaced, curved reflector plates, each positioned around a portion of the outside surface of the lamp adjacent a respective end. The reflector plate aids in slightly increasing the light intensity at the ends of the lamp. While some improvement is provided by the reflector plates, there remains substantial non-uniformity along the surface of the lamp.